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Abstract:

The invention relates to pulse width modulator system (PWMS) comprising a
modulator system input (MI), a modulator output (MO), an amplitude
distribution filter (ADF), and a pulse width modulator (PMOD), wherein
said amplitude distribution filter (ADF) establishes an intermediate
output signal (OS) by modifying the level of the amplitude distribution
of an input signal (IS) within at least one predetermined amplitude range
of said input signal (IS), said input signal (IS) being received from
said modulator system input (MI), and wherein said pulse width modulator
(PMOD) provides a modulator output signal (MOS) on said modulator output
(MO) on the basis of said intermediate output signal (OS).

2. Pulse width modulator system according to claim 1, wherein said pulse
width modulator system (PWMS) is a distributed system.

3. Pulse width modulator system according to claim 1 or claim 2, wherein
said pulse width modulator system (PWMS) comprises an amplifier (AMP),
comprising an amplifier output (AMPO), said amplifier producing an
amplified modulator system output signal (MSOS) on said amplifier output
(AMPO) responsive to said modulator output signal (MOS).

4. Pulse width modulator system according to any of the claims 1 to 3,
wherein said amplifier (AMP) is a switching amplifier.

5. Pulse width modulator system according to any of the claim 1 to 4,
wherein said input signal (IS) is a continuous time signal.

6. Pulse width modulator system according to any of the claim 1 to 5,
wherein said input signal (IS) is a discrete time signal.

7. Pulse width modulator system according to any of the claim 1 to 6,
wherein said input signal (IS) is an audio signal.

8. Pulse width modulator system according to any of the claim 1 to 7,
wherein said modulator output signal (MOS) is a pulse width modulated
signal.

9. Pulse width modulator system according to any of the claim 1 to 8,
wherein said pulse width modulated signal comprises at least partly
curved or inclined pulses.

10. Pulse width modulator system according to any of the claim 1 to 9,
wherein said modulator output signal (MOS) is a three level pulse width
modulated signal.

11. Pulse width modulator system according to any of the claim 1 to 10,
wherein said at least one predetermined amplitude range is dynamically
positioned.

12. Pulse width modulator system according to any of the claim 1 to 11,
wherein the extent of said at least one predetermined amplitude range is
dynamically adapted.

13. Pulse width modulator system according to any of the claim 1 to 12,
wherein said at least one predetermined amplitude range comprises the
zero-level.

14. Pulse width modulator system according to any of the claim 1 to 13,
wherein said at least one predetermined amplitude range is symmetric
around the zero-level.

15. Pulse width modulator system according to any of the claim 1 to 14,
wherein said at least one predetermined amplitude range comprises a range
from a low threshold (LT) up to but not including the zero-level, and a
range from but not including the zero-level up to a high threshold (HT).

16. Pulse width modulator system according to any of the claim 1 to 15,
wherein said at least one predetermined amplitude range does not comprise
the zero-level.

17. Pulse width modulator system according to any of the claim 1 to 16,
wherein said at least one predetermined amplitude range is determined on
the basis of a minimum pulse width (MP) of said modulator output signal
(MOS).

18. Pulse width modulator system according to any of the claim 1 to 17,
wherein said minimum pulse width (MP) of said modulator output signal
(MOS) is determined on the basis of characteristics of said amplifier
(AMP).

19. Pulse width modulator system according to any of the claim 1 to 18,
wherein said characteristics of said amplifier (AMP) comprises the rise
time and/or fall time.

20. Pulse width modulator system according to any of the claim 1 to 19,
wherein said at least one predetermined amplitude range is adaptively
determined on the basis of an external control signal (ECS).

21. Pulse width modulator system according to any of the claim 1 to 20,
wherein said external control signal is provided by an instance of said
pulse width modulator system (PWMS).

22. Pulse width modulator system according to any of the claim 1 to 21,
wherein said amplitude distribution filter (ADF) comprises a signal
generator (SG) adapted for establishing an outband signal (OBS).

23. Pulse width modulator system according to any of the claim 1 to 22,
wherein said amplitude distribution filter (ADF) comprises a level
controlled generator (LCG) adapted for establishing an outband signal
(OBS) at least partly controlled on the basis of the amplitude of said
input signal (IS).

24. Pulse width modulator system according to any of the claim 1 to 23,
wherein the peak amplitude of said outband signal (OBS) varies with the
amplitude of said input signal (IS).

25. Pulse width modulator system according to any of the claim 1 to 24,
wherein the peak amplitude of said outband signal (OBS) at least partly
corresponds to the sum of the level of said input signal (IS) and a safe
offset value (SO).

26. Pulse width modulator system according to any of the claim 1 to 25,
wherein said safe offset value (SO) corresponds to at least the half of
said predetermined amplitude range.

27. Pulse width modulator system according to any of the claim 1 to 26,
wherein the peak amplitude of said outband signal (OBS) for a first input
signal amplitude range at least partly corresponds to the sum of the
amplitude of said input signal (IS) and said safe offset value (SO), for
a second input signal level range at least partly corresponds to the
difference between a predetermined level and the level of said input
signal (IS), and for a third input signal level range at least partly
corresponds to zero.

28. Pulse width modulator system according to any of the claim 1 to 27,
wherein said outband signal (OBS) is a periodic signal.

29. Pulse width modulator system according to any of the claim 1 to 28,
wherein said outband signal (OBS) comprises a frequency corresponding to
the half of the modulation frequency of said pulse width modulator
(PMOD).

30. Pulse width modulator system according to any of the claim 1 to 29,
wherein said outband signal (OBS) comprises a frequency corresponding to
the quarter of the modulation frequency of said pulse width modulator
(PMOD).

31. Pulse width modulator system according to any of the claim 1 to 30,
wherein the wave form of said outband signal (OBS) is a sine waveform, a
triangular waveform, a square waveform or a sawtooth waveform.

32. Pulse width modulator system according to any of the claim 1 to 31,
wherein said outband signal (OBS) is a multiple frequency signal.

33. Pulse width modulator system according to any of the claim 1 to 32,
wherein said outband signal (OBS) comprises the broadest possible
frequency band.

34. Pulse width modulator system according to any of the claim 1 to 33,
wherein the level of the frequency distribution of said outband signal
(OBS) is at least partly reduced within the frequency range comprising
the input signal (IS).

35. Pulse width modulator system according to any of the claim 1 to 34,
wherein said outband signal is at least partly synchronized with the
modulation frequency of said pulse width modulator (PMOD).

36. Pulse width modulator system according to any of the claim 1 to 35,
wherein said synchronization comprises synchronizing the peaks of said
outband signal (OBS) with the modulation frequency and modulation phase
of said pulse width modulator (PMOD).

37. Pulse width modulator system according to any of the claim 1 to 36,
wherein said pulse width modulator (PMOD) comprises a reference signal
representation generator (15) establishing a reference signal
representation.

38. Pulse width modulator system according to any of the claim 1 to 37,
wherein said synchronization comprises synchronizing the outband signal
(OBS) with said reference signal representation of said pulse width
modulator (PMOD).

39. Pulse width modulator system according to any of the claim 1 to 38,
wherein said amplitude distribution filter (ADF) establishes said
intermediate output signal (OS) at least partly by means of an outband
signal summing point (61, 102, 142, 182, 222) adapted for adding said
outband signal to said input signal (IS).

40. Pulse width modulator system according to any of the claim 1 to 39,
wherein said amplitude distribution filter (ADF) further comprises a
noise shaper performing noise shaping of said intermediate output signal
(OS), preferably errors introduced by the addition of said outband signal
(OBS).

41. Pulse width modulator system according to any of the claim 1 to 40,
wherein said noise shaper comprises a loop filter (LF), a filter summing
point (63, 82, 103, 143, 162, 183, 223) and a feedback summing point (61,
81, 101, 141, 161, 181, 221) establishing a feedback path from said
intermediate output signal (OS) to said input signal (IS).

42. Pulse width modulator system according to any of the claim 1 to 41,
wherein said loop filter (LF) comprises a low pass filter, preferably a
low-pass infinite impulse response filter with a corner frequency at
preferably 20 kHz.

43. Pulse width modulator system according to any of the claim 1 to 42,
wherein said amplitude distribution filter (ADF) establishes said
intermediate output signal (OS) at least partly by means of mapping means
(MM) adapted for establishing a mapping output (83, 163) corresponding to
said input signal (IS) in such a way that input signal amplitude
instances that fall within said at least one predetermined amplitude
range are mapped to mapping output amplitude instances according to a
predetermined mapping scheme.

44. Pulse width modulator system according to any of the claim 1 to 43,
wherein said predetermined mapping scheme comprises mapping input signal
amplitude instances that fall within said at least one predetermined
amplitude range to amplitude instances outside said at least one
predetermined amplitude range.

45. Pulse width modulator system according to any of the claim 1 to 44,
wherein said predetermined mapping scheme comprises mapping input signal
amplitude instances that fall within said at least one predetermined
amplitude range to the zero-level.

46. Pulse width modulator system according to any of the claim 1 to 45,
wherein said predetermined mapping scheme is dynamically adapted.

47. Pulse width modulator system according to any of the claim 1 to 46,
wherein said predetermined mapping scheme is controlled by an external
control signal (ECS).

48. Pulse width modulator system according to any of the claim 1 to 47,
wherein said noise shaper further performs noise shaping of errors
introduced by said mapping means (MM).

49. Pulse width modulator system according to any of the claim 1 to 48,
wherein said amplitude distribution filter (ADF) further comprises a
quantizer (QZ).

50. Pulse width modulator system according to any of the claim 1 to 49,
wherein said noise shaper further performs noise shaping of errors
introduced by said quantizer (QZ).

51. Pulse width modulator system according to any of the claim 1 to 50,
wherein said modifying the level of the amplitude distribution of said
input signal (IS) within said at least one predetermined amplitude range
comprises decreasing said level of the amplitude distribution of said
input signal (IS) within said at least one predetermined amplitude range.

52. Pulse width modulator system according to any of the claim 1 to 51,
wherein said modifying the level of the amplitude distribution of said
input signal (IS) within said at least one predetermined amplitude range
comprises entirely restraining said level of the amplitude distribution
of said input signal (IS) within said at least one predetermined
amplitude range.

53. Pulse width modulator system according to any of the claim 1 to 52,
wherein said pulse width modulator (PMOD) comprises an intersection
computing block (12).

54. Pulse width modulator system according to any of the claim 1 to 53,
wherein said pulse width modulator (PMOD) comprises a pulse generator
(14).

55. Pulse width modulator system (PWMS) comprisinga modulator system input
(MI),a modulator output (MO),an amplitude distribution filter (ADF), anda
pulse width modulator (PMOD),wherein said amplitude distribution filter
(ADF) establishes an intermediate output signal (OS) by modifying the
level of the amplitude distribution of an input signal (IS) within at
least one predetermined amplitude range of said input signal (IS), said
input signal (IS) being received from said modulator system input
(MI),wherein said pulse width modulator (PMOD) provides a modulator
output signal (MOS) on said modulator output (MO) on the basis of said
intermediate output signal (OS), andwherein said modifying the level of
the amplitude distribution of an input signal (IS) within at least one
predetermined amplitude range of said input signal (IS) comprises
substantially preventing said pulse width modulator (PMOD) from
establishing pulse width modulation pulses of a width less than a
predetermined minimum pulse width.

56. Pulse width modulator system according to claim 55, wherein said
system further comprises features according to any of the claims 1 to 54.

Description:

[0002]U.S. Pat. No. 5,617,058 discloses a method of minimizing
nonlinearities when amplifying pulse width modulated (PWM) signals. A
problem of the disclosed invention is that it requires complex and power
consuming compensation to be performed in the pulse width modulator. A
further problem is that the compensation performed causes a worsening of
the electromagnetic compatibility and interference (EMC/EMI) properties.

[0003]It is an object of the present invention to provide a pulse width
modulator dealing with nonlinearities of, e.g., amplifiers for pulse
width modulated signals in a cost-effective way.

[0004]It is a further object of the present invention to provide a pulse
width modulator with improved electromagnetic compatibility and
interference (EMC/EMI) properties.

SUMMARY OF THE INVENTION

[0005]The invention relates to a pulse width modulator system PWMS
comprising [0006]a modulator system input MI, [0007]a modulator output
MO, [0008]an amplitude distribution filter ADF, and [0009]a pulse width
modulator PMOD,wherein said amplitude distribution filter ADF establishes
an intermediate output signal OS by modifying the level of the amplitude
distribution of an input signal IS within at least one predetermined
amplitude range of said input signal IS, said input signal IS being
received from said modulator system input MI, and wherein said pulse
width modulator PMOD provides a modulator output signal MOS on said
modulator output MO on the basis of said intermediate output signal OS.

[0010]According to the present invention, an advantageous means for
modifying a signal to better allow for processing or other kinds of
handling by means that do not process all possible signal amplitudes
linearly is provided. By means of the present invention, the
concentration of signal amplitudes that may be problematic to handle
properly by a subsequent means is reduced.

[0011]The present invention is especially beneficial when used with
connection to PWM amplifiers of other uses of pulse width modulation, as
too narrow PWM pulses may often be handled improperly. By means of the
present invention it is possible to prevent too narrow pulses to be
established, or at least reduce the concentration of such pulses.

[0012]The present invention may however as well be used for preventing or
reducing the concentration of pulses of any predetermined pulse widths.
Also in systems where the end product is not pulses, the present
invention may be used for preventing or reducing the concentration of
occurrences within predetermined ranges.

[0013]According to the present invention the term predetermined is to be
understood in a broad sense, thus not necessarily corresponding to the
term fixed. Hence, e.g., the manufacturer or user may actually fix the
predetermined amplitude range, or it may be determined from time to time
during use, e.g. by adapting to changing environmental or internal
parameters, or due to external control.

[0014]Also the concept of pulse width modulation is to be understood in a
broad sense referring to any possible category of pulse width modulation,
comprising, e.g., by sampling method: naturally sampled PWM and uniformly
sampled PWM, by signal parts: single sided PWM and double sided PWM, and
by encoding: two-level PWM, three-level PWM, etc.

[0015]According to the present invention, the pulse width modulator PMOD
may be any kind of means suitable for establishing a pulse width
modulated representation of a signal. Which kind of pulse width modulator
to use in a particular application depends on the type of input signal,
the desired precision and signal/error-ratio, as well as the desired type
of PWM to be established. The pulse width modulator may be of a type
operating on the basis of a reference signal, a reference signal
representation, i.e. a model of a reference signal, or no reference
signal at all. The pulse width modulator may be implemented as an
all-analog circuit, all-digital circuit, software executable, or any kind
of hybrid implementation.

[0016]When said pulse width modulator system PWMS is a distributed system,
an advantageous embodiment of the present invention has been obtained.

[0017]According to the present invention the different system parts may be
physically implemented within a single package, e.g. an integrated
circuit, but they may as well be implemented as individual building
blocks, whether as integrated circuits or by means of discrete
components. Hence the amplitude distribution filter ADF may be positioned
anywhere in the input signal path, and not necessarily in connection with
the pulse width modulator and probably a PWM amplifier. This feature is
made possible by the present invention offering pre-processing of a
signal that at some point are to be processed by means comprising
non-linear processing within certain amplitude ranges. Hence also setups
comprising further processing blocks between the amplitude distribution
filter ADF and the pulse width modulator PMOD is within the scope of the
present invention.

[0018]When said pulse width modulator system PWMS comprises an amplifier
AMP, comprising an amplifier output AMPO, said amplifier producing an
amplified modulator system output signal MSOS on said amplifier output
AMPO responsive to said modulator output signal MOS, an advantageous
embodiment of the present invention has been obtained.

[0019]By amplifying the modulator output signal MOS is typically referred
to increasing the amplitude of the PWM pulses. According to the present
invention any kind of amplifier may be used for amplifying the PWM
signal.

[0020]When according to an embodiment of the invention said amplifier AMP
is a switching amplifier, an advantageous embodiment of the present
invention has been obtained.

[0021]As generally appreciated, switching amplifiers are the most
cost-effective means for amplifying a PWM signal. According to the
present invention any kind of switching amplifier may be used, e.g.
simple electrically or electronically switches, power electronic
switches, e.g. power transistors, complex switch setups such as
h-bridges, etc.

[0022]When said input signal IS is a continuous time signal, an
advantageous embodiment of the present invention has been obtained.

[0023]According to the present invention, the input signal may be a
continuous time signal, e.g. from the output of an analog audio media
reading means or any other kind of analog and/or continuous time signal.

[0024]When said input signal IS is a discrete time signal, an advantageous
embodiment of the present invention has been obtained.

[0025]According to the present invention, the input signal is preferably a
discrete time signal, such as e.g. a sampled signal or the output of a
true digital audio processing means. The input signal may e.g. be a pulse
code modulated (PCM) signal.

[0026]When said input signal IS is an audio signal, an advantageous
embodiment of the present invention has been obtained.

[0027]According to the present invention, the utility content of the input
signal is preferably audio of any kind, i.e. representing a utility band
of 20 kHz, preferably 48 kHz, and even more preferably 96 kHz.

[0028]When said modulator output signal MOS is a pulse width modulated
signal, an advantageous embodiment of the present invention has been
obtained.

[0029]According to the present invention, the modulator output signal MOS
established by the pulse width modulator PMOD is preferably a PWM signal.

[0030]When said pulse width modulated signal comprises at least partly
curved or inclined pulses, an advantageous embodiment of the present
invention has been obtained.

[0031]In order to further improve the electromagnetic compatibility and
interference (EMC/EMI) properties, non-square shapes may be used for
establishing the PWM pulses.

[0032]When said modulator output signal MOS is a three level pulse width
modulated signal, an advantageous embodiment of the present invention has
been obtained.

[0033]According to the present invention the PWM signal is preferably a
three level PWM signal, and preferably encoded on two signal carriers
whereof a first carries pulses representing positive input signal
amplitudes and a second carries pulses representing negative input signal
amplitudes whereby the two signal carriers preferably never
simultaneously comprise pulses. Any kind of three level PWM signals may
however be used with the present invention, as well as two level or
multiple level signals.

[0034]When said at least one predetermined amplitude range is dynamically
positioned, an advantageous embodiment of the present invention has been
obtained.

[0035]According to the present invention, the predetermined amplitude
range may be moved among the possible signal amplitudes during use.
Thereby it is possible to adapt the system of the present invention to
actual environmental or internal parameter changes, as well as to let the
system respond to externally provided control information.

[0036]When the extent of said at least one predetermined amplitude range
is dynamically adapted, an advantageous embodiment of the present
invention has been obtained.

[0037]According to the present invention, the extent, i.e. the values of
the low and high thresholds LT, HT, relative to each other, of the
predetermined amplitude range may be changed during use. Thereby it is
possible to adapt the system of the present invention to actual
environmental or internal parameter changes, as well as to let the system
respond to externally provided control information.

[0038]When said at least one predetermined amplitude range comprises the
zero-level, an advantageous embodiment of the present invention has been
obtained.

[0039]According to the present invention, the predetermined amplitude
range preferably comprises the zero-level, as it is around that level
most PWM amplifiers process data non-linearly.

[0040]When said at least one predetermined amplitude range is symmetric
around the zero-level, an advantageous embodiment of the present
invention has been obtained.

[0041]According to the present invention, the predetermined amplitude
range is preferably symmetric around the zero-level, as this is one of
the typical properties of most PWM amplifiers' non-linear processing
range.

[0042]When said at least one predetermined amplitude range comprises a
range from a low threshold LT up to but not including the zero-level, and
a range from but not including the zero-level up to a high threshold HT,
an advantageous embodiment of the present invention has been obtained.

[0043]According to the present invention, the exact zero-level is often
not included in the non-linear processing range of a processing means,
even if both sides of it are. Hence a preferred embodiment of the present
invention that utilizes this fact is provided.

[0044]When said at least one predetermined amplitude range does not
comprise the zero-level, an advantageous embodiment of the present
invention has been obtained.

[0045]According to the present invention it is possible to deal with
predetermined amplitude ranges not comprising the zero-level.

[0046]When said at least one predetermined amplitude range is determined
on the basis of a minimum pulse width MP of said modulator output signal
MOS, an advantageous embodiment of the present invention has been
obtained.

[0047]According to the present invention, a preferred embodiment is
obtained by determining the predetermined amplitude range on the minimum
width of the PWM pulses in the modulator output signal MOS.

[0048]When said minimum pulse width MP of said modulator output signal MOS
is determined on the basis of characteristics of said amplifier AMP, an
advantageous embodiment of the present invention has been obtained.

[0049]According to the present invention, an even more preferred
embodiment is obtained by determined the minimum pulse widths on the
basis of characteristics of the amplifier.

[0050]When said characteristics of said amplifier AMP comprises the rise
time and/or fall time, an advantageous embodiment of the present
invention has been obtained.

[0051]Due to the power electronics typically used within a PWM amplifier,
rise and fall times of the switches become significant compared to the
desired PWM pulse precision, i.e. the positioning of the flanks as well
as the exact width of each pulse. By determining the minimum pulse width
on the basis of the rise and/or fall time of the pulses established by
the amplifier, a preferred embodiment of the present invention has been
obtained.

[0052]When said at least one predetermined amplitude range is adaptively
determined on the basis of an external control signal ECS, an
advantageous embodiment of the present invention has been obtained.

[0053]According to the present invention is it possible, in addition to or
instead of determining a predetermined amplitude range on the basis of
amplifier characteristics, to determine the amplitude ranges on the basis
of control information obtained from externally. Thus an embodiment of
the present invention allows, e.g., an external control unit to
dynamically adapt the predetermined amplitude ranges.

[0054]When said external control signal is provided by an instance of said
pulse width modulator system PWMS, an advantageous embodiment of the
present invention has been obtained.

[0055]For example a multiple channel PWM amplifier comprising several
pulse width modulator systems according to the present invention, uses a
shared control unit to dynamically adapt the predetermined amplitude
ranges of each modulator system in order to avoid concurrent flanks in
the different channels. Thereby, e.g., cross talk is reduced.

[0056]When said amplitude distribution filter ADF comprises a signal
generator SG adapted for establishing an outband signal OBS, an
advantageous embodiment of the present invention has been obtained.

[0057]When said amplitude distribution filter ADF comprises a level
controlled generator LCG adapted for establishing an outband signal OBS
at least partly controlled on the basis of the amplitude of said input
signal IS, an advantageous embodiment of the present invention has been
obtained.

[0058]According to the present invention, characteristics of the outband
signal are controlled on the basis of the amplitude of the input signal.
Thereby it is possible to adapt the amplitude distribution filter to a
predetermined range by comparing a current input signal amplitude with
the predetermined amplitude range or any other set of thresholds, and
establish the outband signal differently depending on the comparison
results.

[0059]When the peak amplitude of said outband signal OBS varies with the
amplitude of said input signal IS, an advantageous embodiment of the
present invention has been obtained.

[0060]According to the present invention, the outband signal amplitude
varying with the input signal amplitude is to be understood in a loose
sense, as this does not necessarily mean that the amplitudes follows each
other by a fixed offset. The correspondence between the outband signal
amplitude and the input signal amplitude is preferably changed according
to the input signal amplitude.

[0061]When the peak amplitude of said outband signal OBS at least partly
corresponds to the sum of the level of said input signal IS and a safe
offset value SO, an advantageous embodiment of the present invention has
been obtained.

[0062]According to the present invention, the outband signal amplitude in
an embodiment varies tightly with the input signal amplitude, as it is
determined only by adding an offset to the input signal amplitude.

[0063]When said safe offset value SO corresponds to at least the half of
said predetermined amplitude range, an advantageous embodiment of the
present invention has been obtained.

[0064]Thereby is ensured that no intermediate output signal amplitude
peaks is within the predetermined amplitude range.

[0065]When the peak amplitude of said outband signal OBS for a first input
signal amplitude range at least partly corresponds to the sum of the
amplitude of said input signal IS and said safe offset value SO, for a
second input signal level range at least partly corresponds to the
difference between a predetermined level and the level of said input
signal IS, and for a third input signal level range at least partly
corresponds to zero, an advantageous embodiment of the present invention
has been obtained.

[0066]By amplitude modulating the outband signal differently for different
input signal amplitude range may e.g. better adapt to the restrictions of
the system. By ramping in and out of the ranges instead of using hard
changes may improve the signal/error ratio.

[0067]When said outband signal OBS is a periodic signal, an advantageous
embodiment of the present invention has been obtained.

[0068]When simplicity and cost-effectiveness is considered a priority, the
outband signal should preferably be a periodic signal, due to the easy
establishment of such a signal or signal representation.

[0069]When said outband signal OBS comprises a frequency corresponding to
the half of the modulation frequency of said pulse width modulator PMOD,
an advantageous embodiment of the present invention has been obtained.

[0070]By choosing an outband signal frequency of half the PWM frequency it
may for several simple outband signal wave form types be ensured that the
outband signal amplitude peaks alternates between a high and a low peak
with one such peak for each PWM period. When that situation is
established, the performance of the amplitude distribution filter is
improved.

[0071]When said outband signal OBS comprises a frequency corresponding to
the quarter of the modulation frequency of said pulse width modulator
PMOD, an advantageous embodiment of the present invention has been
obtained.

[0072]By choosing an outband signal frequency of the quarter of the PWM
frequency it may for several simple outband signal wave form types be
ensured that the intermediate output signal amplitude for each four PWM
periods has the values: high peak, zero, low peak, zero. When that
situation is established, the performance of the amplitude distribution
filter is improved, as well as a reduction of the power dissipation in
the amplifier due to the absence of pulses on zero-level input.

[0073]When the wave form of said outband signal OBS is a sine waveform, a
triangular waveform, a square waveform or a sawtooth waveform, an
advantageous embodiment of the present invention has been obtained.

[0074]According to the present invention, simple periodic signals are
preferred in order to prioritise simple and cost-effective
implementation.

[0075]When said outband signal OBS is a multiple frequency signal, an
advantageous embodiment of the present invention has been obtained.

[0076]According to the present invention, an outband signal comprising
multiple frequencies is preferred in order to prioritise electromagnetic
compatibility and interference (EMC/EMI) properties, as well as reducing
the errors introduced when the outband signal is correlated to the input
signal.

[0077]When said outband signal OBS comprises the broadest possible
frequency band, an advantageous embodiment of the present invention has
been obtained.

[0078]The broader frequency band comprised by the outband signal, the
better EMC/EMI properties.

[0079]When the level of the frequency distribution of said outband signal
OBS is at least partly reduced within the frequency range comprising the
input signal IS, an advantageous embodiment of the present invention has
been obtained.

[0080]In order to avoid introduction of errors in the utility band, the
frequency spectrum of the outband signal should preferably be shaped by
reducing the utility band concentration.

[0081]When said outband signal is at least partly synchronized with the
modulation frequency of said pulse width modulator PMOD, an advantageous
embodiment of the present invention has been obtained.

[0082]In order to optimize the performance of the amplitude distribution
filter the outband signal should preferably be synchronized with the PWM
frequency. Such synchronization may comprise direct synchronization
between the outband signal and the PWM frequency or it may comprise
indirect synchronization where the outband signal is established in such
a way that the intermediate output signal becomes synchronized with the
PWM frequency.

[0083]When said synchronization comprises synchronizing the peaks of said
outband signal OBS with the modulation frequency and modulation phase of
said pulse width modulator PMOD, an advantageous embodiment of the
present invention has been obtained.

[0084]By ensuring, e.g., that the outband signal amplitude peaks, or
preferably the intermediate output signal amplitude peaks, alternates
between a high and a low peak with one such peak for each PWM period, or
that the outband signal amplitude, or preferably the intermediate output
signal amplitude, for each four PWM periods has the values: high peak,
zero, low peak, zero, the performance of the amplitude distribution
filter is improved.

[0085]By synchronization with the modulation phase is referred to
synchronizing the outband peaks with the times in each PWM period where
the input signal is considered or sampled, or more preferably
establishing an outband signal that causes the intermediate output signal
peaks to be synchronized with the times in each PWM period where the
signal is considered or sampled by the pulse width modulator PMOD.

[0087]According to the present invention, the pulse width modulator may
determine the times of the PWM pulse flanks by comparing the utility
signal with a reference signal representation. This reference signal
representation may be a physical signal or a model of a such comprising
polynomials or a table of values.

[0088]When said synchronization comprises synchronizing the outband signal
OBS with said reference signal representation of said pulse width
modulator PMOD, an advantageous embodiment of the present invention has
been obtained.

[0089]When the pulse width modulator operates on a reference signal, it
may be beneficial to synchronize the outband signal with that reference
signal representation. Such synchronization may comprise direct
synchronization between the outband signal and the reference signal or it
may comprise indirect synchronization where the outband signal is
established in such a way that the intermediate output signal becomes
synchronized with the reference signal.

[0090]When said amplitude distribution filter ADF establishes said
intermediate output signal OS at least partly by means of an outband
signal summing point 61, 102, 142, 182, 222 adapted for adding said
outband signal to said input signal IS, an advantageous embodiment of the
present invention has been obtained.

[0091]By adding the preferably high frequency outband signal to the
utility signal is ensured that it may easily be retracted by, e.g., low
pass filtering, as is, e.g., performed by a demodulator DEM, without
disturbing the utility signal.

[0092]When said amplitude distribution filter ADF further comprises a
noise shaper performing noise shaping of said intermediate output signal
OS, preferably errors introduced by the addition of said outband signal
OBS, an advantageous embodiment of the present invention has been
obtained.

[0093]By adding a noise shaper is ensured that the errors introduced are
moved into frequency bands where they are later easily removed by e.g.
demodulation.

[0095]According to the present invention, the above described noise shaper
embodiment is preferred. It is however noted that any kind of noise
shaper implementation is within the scope of the present invention.

[0096]When said loop filter LF comprises a low pass filter, preferably a
low-pass infinite impulse response filter with a corner frequency at
preferably 20 kHz, an advantageous embodiment of the present invention
has been obtained.

[0097]According to the present invention, the above described loop filter
embodiment is preferred. It is however noted that any kind of loop filter
implementation is within the scope of the present invention. Also a
distribution of parts the loop filter or a relocation of the whole loop
filter to different positions in the circuit is within the scope of the
present invention.

[0098]When said amplitude distribution filter ADF establishes said
intermediate output signal OS at least partly by means of mapping means
MM adapted for establishing a mapping output 83, 163 corresponding to
said input signal IS in such a way that input signal amplitude instances
that fall within said at least one predetermined amplitude range are
mapped to mapping output amplitude instances according to a predetermined
mapping scheme, an advantageous embodiment of the present invention has
been obtained.

[0099]According to the present invention the mapping means may be any
suitable means for performing the described mapping, whether analog,
digital or software based. Such means may, e.g., comprise table lookups,
"if-else" conditional structures, "switch-case" conditional structures or
any other suitable means.

[0100]When said predetermined mapping scheme comprises mapping input
signal amplitude instances that fall within said at least one
predetermined amplitude range to amplitude instances outside said at
least one predetermined amplitude range, an advantageous embodiment of
the present invention has been obtained.

[0101]According to the present invention, input signal amplitudes within
the predetermined amplitude range are in the mapping output 83, 163
mapped to amplitudes outside the predetermined amplitude range. Thereby
is for a PWM system ensured that no pulses with a width of less than the
minimum pulse width are established.

[0102]When said predetermined mapping scheme comprises mapping input
signal amplitude instances that fall within said at least one
predetermined amplitude range to the zero-level, an advantageous
embodiment of the present invention has been obtained.

[0103]According to the present invention, input signal amplitudes within
the predetermined amplitude range are in the mapping output 83, 163
mapped to the zero-level, which may cause several PWM-periods to comprise
no pulse. This may in certain applications cause a considerable saving on
amplifier power consumption.

[0104]When said predetermined mapping scheme is dynamically adapted, an
advantageous embodiment of the present invention has been obtained.

[0105]According to the present invention, the predetermined mapping scheme
may be dynamically adapted, preferably in order to adapt to changes in
the predetermined amplitude range. Thereby it is possible to adapt the
system of the present invention to actual environmental or internal
parameter changes, as well as to let the system respond to externally
provided control information.

[0106]When said predetermined mapping scheme is controlled by an external
control signal ECS, an advantageous embodiment of the present invention
has been obtained.

[0107]According to the present invention is it possible, in addition to or
instead of determining a predetermined mapping scheme on the basis of
amplifier characteristics, to determine the mapping scheme on the basis
of control information obtained from externally. Thus an embodiment of
the present invention allows, e.g., an external control unit to
dynamically adapt the predetermined mapping scheme.

[0108]When said noise shaper further performs noise shaping of errors
introduced by said mapping means MM, an advantageous embodiment of the
present invention has been obtained.

[0109]By adding a noise shaper is ensured that the errors introduced are
moved into frequency bands where they are later easily removed by e.g.
demodulation.

[0110]When said amplitude distribution filter ADF further comprises a
quantizer QZ, an advantageous embodiment of the present invention has
been obtained.

[0111]As quantization of the utility signal is often necessary or
preferred before or in connection with the pulse width modulation, a
quantizer is implemented in a preferred embodiment. By placing the
quantizer within the amplitude distribution filter, the noise shaping
capabilities of preferred embodiments are also applied to the
quantization errors inherently introduced by any quantization.

[0112]When said noise shaper further performs noise shaping of errors
introduced by said quantizer QZ, an advantageous embodiment of the
present invention has been obtained.

[0113]By adding a noise shaper is ensured that the errors introduced are
moved into frequency bands where they are later easily removed by e.g.
demodulation.

[0114]When said modifying the level of the amplitude distribution of said
input signal IS within said at least one predetermined amplitude range
comprises decreasing said level of the amplitude distribution of said
input signal IS within said at least one predetermined amplitude range,
an advantageous embodiment of the present invention has been obtained.

[0115]By reducing the concentration of certain intermediate output signal
amplitude ranges without causing harmful alteration of the utility signal
an exceptionally useful embodiment has been obtained, allowing a
pre-conditioning of a signal to better fit to a processing stage
comprising problematic processing within these certain amplitude ranges.

[0116]When said modifying the level of the amplitude distribution of said
input signal IS within said at least one predetermined amplitude range
comprises entirely restraining said level of the amplitude distribution
of said input signal IS within said at least one predetermined amplitude
range, an advantageous embodiment of the present invention has been
obtained.

[0117]By substantially preventing certain amplitude ranges to occur within
an intermediate output signal without causing harmful alteration of the
utility signal an exceptionally useful embodiment has been obtained,
allowing a pre-conditioning of a signal to better fit to a processing
stage comprising problematic processing within these certain amplitude
ranges.

[0118]When said pulse width modulator PMOD comprises an intersection
computing block 12, an advantageous embodiment of the present invention
has been obtained.

[0119]According to the present invention, the intersection computing block
may be any suitable means for measuring, monitoring, estimating,
computing, etc., the times of intersection between two signals. Such
means may, e.g., comprise comparators, logical gates or software-based
routines, or any other suitable implementation.

[0120]When said pulse width modulator PMOD comprises a pulse generator 14,
an advantageous embodiment of the present invention has been obtained.
According to the present invention the pulse generator may be any means
suitable for establishing PWM pulses.

[0121]The present invention further relates to a pulse width modulator
system PWMS comprising [0122]a modulator system input MI, [0123]a
modulator output MO, [0124]an amplitude distribution filter ADF, and
[0125]a pulse width modulator PMOD,wherein said amplitude distribution
filter ADF establishes an intermediate output signal OS by modifying the
level of the amplitude distribution of an input signal IS within at least
one predetermined amplitude range of said input signal IS, said input
signal IS being received from said modulator system input MI, wherein
said pulse width modulator PMOD provides a modulator output signal MOS on
said modulator output MO on the basis of said intermediate output signal
OS, and wherein said modifying the level of the amplitude distribution of
an input signal IS within at least one predetermined amplitude range of
said input signal IS comprises substantially preventing said pulse width
modulator PMOD from establishing pulse width modulation pulses of a width
less than a predetermined minimum pulse width.

[0126]When said system further comprises any of the above-mentioned
features, an advantageous embodiment of the present invention has been
obtained.

THE DRAWINGS

[0127]The invention will in the following be described with reference to
the drawings where

[0128]FIG. 1A illustrates an embodiment of the present invention,

[0129]FIG. 1B illustrates an embodiment of the present invention shown in
a context,

[0177]FIG. 24 illustrates an embodiment of a multi-channel pulse width
modulator system MCS.

DETAILED DESCRIPTION

[0178]FIG. 1A illustrates an embodiment of a pulse width modulator system
PWMS of the present invention. It comprises a modulator input MI
receiving an input signal IS. The input signal may is preferably the
utility signal to be pulse width modulated. An amplitude distribution
filter ADF of the present invention processes the input signal IS in
order to adapt the input signal amplitude distribution to achieve the
best performance of subsequent stages. The amplitude distribution filter
ADF establishes an output signal OS, also referred to as intermediate
output signal, which is fed to a pulse width modulator PMOD. The pulse
width modulator PMOD establishes a pulse width modulated representation
of the output signal, and outputs it via a modulator output MO as a
modulator output signal MOS.

[0179]FIG. 1B illustrates a further embodiment of a pulse width modulator
system PWMS of the present invention, and a context for its use. It
comprises all elements of FIG. 1A coupled as described above. It
furthermore comprise an amplifier AMP received the modulator output MOS.
The amplifier is preferably of the power switch type, but may be any
amplifier suited for amplifying a PWM signal. The amplifier may comprise
any number of switches and couplings of these, in accordance with the
type of PWM modulation scheme used by the PWM modulator PMOD. The
amplifier outputs a modulator system output signal MSOS via an amplifier
output AMPO. The modulator system output signal MSOS being the output of
the pulse width modulator system PWMS of the present embodiment is
preferably demodulated by means of a demodulator DEM, typically a
low-loss, low-pass filter, and is fed to a loudspeaker LS. According to
the particular PWM modulation scheme used the amplifier, demodulator and
loudspeaker may be coupled in any way suitable. This particularly applies
for systems where the PWM signal is distributed over two or more
simultaneous signal parts, e.g. as typically used for three level PWM
signals.

[0180]FIG. 1C illustrates a further embodiment of a pulse width modulator
system PWMS of the present invention, and in particular is illustrates an
example of an embodiment of the pulse width modulator PMOD. The
illustrated system comprises a PWM amplifier/audio system for use with a
discrete time input signal, e.g. a pulse code modulated (PCM) signal.

[0181]Like the embodiment of FIG. 1B the present embodiment comprises a
modulator system input MI feeding an input signal IS to an amplitude
distribution filter ADF. The intermediate output signal OS of the
amplitude distribution filter ADF is fed to the pulse width modulator
PMOD.

[0182]The modulator output MOS of the pulse width modulator PMOD, i.e. a
pulse width modulated signal, is amplified by means of an amplifier AMP
as described above and rendered into sound by means of a demodulator DEM
and a loudspeaker setup LS as also described above.

[0183]FIG. 1C further illustrates an embodiment of a pulse width modulator
PMOD. It comprises an upsampling block 11 basically transforming the
intermediate output signal OS from one sampling frequency representation
into an N times higher sampling representation.

[0184]The upsampled signal is then fed to an intersection-computing block
12 adapted for determination of intersections with a parallel reference
signal representation 16 provided by a reference signal generator 15. The
intersections may e.g. be established in the block 12 according to the
principles disclosed in PCT/DK03/00334 hereby incorporated by reference,
or in PCT/DK2004/000361 hereby incorporated by reference. A consecutive
noise shaping and quantizing block 13 feeds the established intersections
to a pulse generator 14 which establishes the modulator output signal
MOS, i.e. a pulse width modulated signal.

[0185]It is noted that the above-described embodiment of a pulse width
modulator PMOD is only one of several possible embodiments suitable for
use with the present invention. Also several different kinds of pulse
width modulation and encoding schemes may be used for establishing the
pulse width modulated signal MOS. This signal may thus perfectly be
distributed over several sub-signals, e.g. when differentiated PWM
signals are established. In such cases also the amplifier AMP may
comprise several sub-amplifiers, typically power switches, and the
demodulator DEM may comprise several demodulators. Also the loudspeaker
setup may comprise several signal inputs.

[0186]An example of a further pulse width modulator PMOD embodiment that
may be used with the present invention is disclosed in PCT/DK03/00475,
hereby incorporated by reference.

[0187]A further example of a pulse width modulator PMOD embodiment that
may be used with the present invention is disclosed in EP 1 178 388 A1,
hereby incorporated by reference.

[0188]It is noted that the above-mentioned embodiment examples are not
exhaustive, and that the present invention may be used in any context for
any application and that the illustrations in FIGS. 1A, 1B and 1C are
only examples for establishing a concept and context for the following
detailed description.

[0189]FIGS. 2A to 2D illustrate one kind of nonlinear processing of input
within certain ranges by a processing stage, which may be compensated for
by means of the present invention. The signals shown in FIGS. 2A-2D
relate to digital amplifiers with power switches.

[0190]FIG. 2A illustrates an analog input signal 21 depicted in a system
of a vertical amplitude axis and a horizontal time axis.

[0191]FIG. 2B illustrates a result of pulse width modulating the signal 21
of FIG. 2A. The pulses 22, 23 are shown relative to the same horizontal
time axis as the signal 21 of FIG. 2A. The time axis comprises a number
of divisions reflecting the periods of the pulse width modulation (PWM),
and for each of these the analog signal amplitude may be decoded from the
width of the corresponding pulse. The pulses shown in FIG. 2B are ideal
so far that they comprise totally vertical flanks. The modulation scheme
used for the PWM signal shown in FIG. 2B is a three-level modulation,
thus representing negative analog signal amplitudes by negative pulses.
Thereby, large amplitude analog signals, either positive or negative, are
represented by wide pulses, positive or negative, and small amplitude
analog signals are represented by narrow pulses. Ideally, a zero input is
represented by the absence of pulses. As a typical analog signal, e.g.
representing audio, crosses the zero-value very often and, furthermore,
also very often stays near zero for longer time periods, several
extremely narrow pulses are established when the signal is pulse width
modulated according to the above-described three-level modulation scheme
and other modulation schemes. Especially because input signals that in
theory should be zero in practice are often alternating around the
zero-level, several very narrow PWM pulses are established by empty input
signals.

[0192]FIG. 2C illustrates a probable output of a set of power switches
applied for amplifying the PWM signal of FIG. 2B. The horizontal time
axis of FIG. 2C corresponds to the time axis of FIG. 2B but the vertical
amplitude axis is, due to clearance, scaled as the pulses of FIG. 2C
probably have larger amplitudes than the pulses of FIG. 2B due to the
amplification. As it takes some time for the power switches to build up
the high voltages on their output, rise and fall times distort the output
pulses 24, 25. However, for PWM pulses 22 of a certain width, the rise
and fall times in combination contribute to a pulse 24 effectively having
the same area, and thereby energy, as the input pulse 22.

[0193]FIG. 2D illustrates the effective output of the power switches
represented by rectangular pulses in order to ease the comparison with
the input pulses of FIG. 2B. Thus, the pulses of FIG. 2D have the widths
of the pulses of FIG. 2B but the area of the pulses of FIG. 2C. These
restrictions leave any differences to be represented by the pulse
heights, thus allowing comparison of the pulse heights of FIG. 2B with
the pulse heights of FIG. 2D.

[0194]Thus, the pulses 26 may represent the effect of the, however
distorted, pulses 24 established from the pulses 22, and it may easily be
seen that apart from the vertical scaling issue according to any
amplification, the effective output pulses 26 equal the input pulses 22.

[0195]As the rise and fall times are independent from the actual input
pulses 22, 23, the same rise and fall times are applied to the narrow
input pulses 23. For input pulses narrower than the rise time, the actual
output pulses, thus, never reaches the full amplitude before falling back
to zero level as illustrated by the output pulses 25. In order to be able
to compare the effect of the output from the narrow pulses, FIG. 2D
comprises the effect of such pulses 27. As seen, the pulse height
decreases the narrower the input pulses are indicating loss of energy
and, thus, introduction of distortion and errors.

[0196]In FIG. 2B the minimum pulse width MP is shown. This width
represents the narrowest pulses that are not distorted, according to this
problem, by the power switches. The minimum pulse width MP is determined
by features of the actual digital amplifier, e.g. power switches, power
supply, voltage regulation, load balancing, etc. Thus, one way to
minimize this problem would be to develop better power switches and
accessories but as the problem is especially significant for input signal
amplitudes close to zero, the problem may never be eliminated. Thus, the
minimum pulse width MP represents an input amplitude range to which the
processing stage, i.e. the amplifier, acts in a non-linear manner.

[0197]However, true zero-level input, i.e. input that is represented by
the absence of pulses, though definitely narrower than the minimum pulse
width MP, may be processed by the power switches and further stages
without distortion. When the non-linear processing range, thus, ranges
across the zero-level, the non-linear processing range does however
typically not comprise the exact value of zero.

[0198]As explained above, ideal PWM signals often comprise several very
narrow pulses caused by an empty or silent input, which is actually not
completely silent in praxis. This typically causes digital amplifiers to
be noisy when the input is silent.

[0199]It is noted that the minimum pulse width constraint does not apply
to all types of pulse width modulated signals, for example is it possible
to realize pulses of any width by means of a conventional differential
PWM technique where two sets of switches operate simultaneously thus
establishing overlapping pulses. Such techniques however typically
provide poor electromagnetic compatibility and interference (EMC/EMI)
properties as well as higher power dissipation.

[0200]FIGS. 3A and 3B illustrate this more generally. FIG. 3A illustrates
by a straight line 31 the common relationship between the output and the
input of a linear process. This relationship is desirable in most
applications processing an input, especially in audio applications, and
as almost no electronical parts or other parts inherently offer linearity
for a wide input range, the linearity requirement typically makes the
development more difficult, the expenses greater and introduces several
other limitations.

[0201]FIG. 3B illustrates a relationship between the output and the input
of a process being non-linear for inputs within a certain amplitude
range, i.e. between a low threshold LT and a high threshold HT. Thus, for
inputs within that amplitude range, the output is distorted. The
relationship illustrated in FIG. 3B may very well be that between the
output and input of a PWM amplifier, as described above, thus introducing
distortion when the input is close to zero. The low threshold LT and high
threshold HT values, thus, correspond to the minimum pulse width MP of
FIG. 2B or more accurately to the analog input signal 21 amplitude
corresponding to the minimum pulse width MP.

[0202]It is noted that the range of input amplitudes where the processing
means, e.g. a pulse width modulator PMOD or an amplifier AMP, performs
non-linear processing according to the present invention need not
necessarily be arranged around the zero level or middle value, but may be
anywhere within the input range, occupy any width of the range, and even
be divided into several non-linear ranges. In the following such ranges
may be referred to as predetermined amplitude ranges or problematic
ranges, which represents amplitude ranges that should be sought prevented
from use due to subsequent non-linear processing or for any other reason.

[0203]FIGS. 4A to 4F are provided for reference purposes and illustrate
characteristics of the output signal OS when the amplitude distribution
filter ADF is empty, i.e. comprises only a wire with a gain of 1 for all
relevant frequencies.

[0204]FIG. 4A illustrates the level distribution of the output signal OS
when the input signal is a 1 kHz sine wave at different input signal peak
levels. The vertical axis corresponds to different peak amplitudes of the
sine wave ranging from 0 to 1. The horizontal axis corresponds to the
possible output signal levels, i.e. -1 to 1. The lightness corresponds to
the level distribution in such a way that darker areas represent higher
concentrations. Thus, any white areas represent combinations of input
peak level and output levels that never occur, whereas dark grey areas
represent combinations that frequently occur. It can, thus, be seen that
for an empty input signal, i.e. a sine wave with an amplitude of zero,
the only value represented in the output signal is also zero, whereas the
output level distribution for an input sine wave of full extend comprises
all possible values and is more concentrated at high absolute amplitudes.

[0205]In order to clarify the meaning of the different shades of grey in
FIG. 4A, FIGS. 4B to 4F are provided. These figures comprise sectional
views of FIG. 4A at different input peak levels. The horizontal axes of
FIGS. 4B to 4F are the same as the horizontal axis in FIG. 4A, i.e.
corresponding to the output range. The vertical axes of FIGS. 4B to 4F
correspond to the output level distribution, i.e. the lightness of FIG.
4A.

[0206]FIG. 4B is a sectional view of FIG. 4A at an input peak level of
zero. As described above, only the value zero is represented in the
output level distribution.

[0207]FIG. 4C is a sectional view of FIG. 4A at an input peak level of
0.25, i.e. a quarter of the full possible extend and, thus, comprises the
output level distribution of a 1 kHz sine wave with a peak value of 0.25.
As seen, the output signal level is distributed over values from -0.25 to
0.25 and is most concentrated at the outer edges due to the slow-varying
nature of a sine wave when at its top or bottom levels.

[0208]FIGS. 4D to 4F also comprise sectional views of FIG. 4A at input
peak levels 0.5, 0.75 and 1.0. Common to all input peak levels, as easily
seen from FIG. 4A, is that the output signal OS comprises levels around
zero as well as all other values between the lower and upper peak levels.
The output signal, thus, comprises values within the nonlinear processing
range of a typical subsequent processing means PM, e.g. a PWM amplifier.

[0209]In the following, with reference to the FIGS. 5A to 22C, embodiments
of the present invention are described in the context of an application
where the signal is pulse width modulated and amplified by a means
performing non-linear processing for input amplitudes within a range
centred around the zero-level as illustrated in FIG. 3B. It is noted that
the present invention may be used in any context where non-linear
processing is performed for certain input amplitude ranges, or the use of
such ranges are sought prevented for any other reason.

[0210]FIG. 5A illustrates an embodiment of the amplitude distribution
filter ADF of the present invention. It comprises an input signal IS and
an output signal OS. The output signal OS is established by adding an
outband signal OBS to the input signal IS. The outband signal is
established by means of a signal generator SG. Dependent of the natures
of the input and outband signals, the level of the amplitude distribution
function of the composite signal OS may be lower within the predetermined
amplitude range than the level of the distribution function of the
unmodified input signal. In other words, the amplitude of the composite
signal OS may be within the predetermined amplitude range for in total a
shorter time than the input signal IS, thus, in the case of subsequent
pulse width modulation, causing fewer pulses narrower than the minimum
pulse width to be established. Also dependent on the characteristics of
the outband signal it may only influence the spectrum of the input signal
without the utility frequency band. For example, when the input signal is
an audio signal of 96 kHz, choosing a high frequency outband signal, e.g.
of 192 kHz, does not disturb the audio content of the signal. According
to the present invention it is, thus, possible to add an outband signal
to the input signal without causing any impact on the utility content of
the signal but yet conditioning the signal to better avoid certain
non-linear or otherwise problematic ranges of subsequent processing
stages.

[0211]The waveform of the outband signal OBS may be anything. As regards
electromagnetic compatibility and interference (EMC/EMI) issues, i.e.
possible problems related to electromagnetic radiation, a waveform with
the widest frequency spectrum possible should be used, thus distributing
the energy over the broadest band possible. Such a possible wide-spectrum
outband signal may comprise white noise shaped to not interfere with the
audio band. In several practical uses, and in the following description
due to clarity, the outband signal OBS is a periodic signal, e.g. a sine
wave, a triangular wave, a square wave, etc., and preferably of a rate
half the rate of the pulse width modulation. Thereby, an outband signal
may be established alternating between its highest and lowest amplitudes
at the times where the pulse width modulator samples the signal.
Furthermore, when the maximum amplitude of the outband signal is outside
the predetermined amplitude range, the outband signal itself never causes
an amplitude value within the non-linear range to be sampled.

[0212]FIGS. 5B and 5E illustrate two possible input signals. In FIG. 5B
the input signal IS is always zero. The horizontal axis corresponds to
the time t or any other time wise division such as, e.g., samples and the
vertical axis represents the amplitude. In the diagram by dashed lines a
high threshold HT and a low threshold LT are, furthermore, shown. This is
the amplitude range in which, e.g., a subsequent amplifier AMP or other
processing means may act in a non-linear way, and the input signal
example of this figure is, thus, within this probably problematic range
all the time. While this input signal is perfect in theory, as it causes
no PWM pulses to be established at all, it may as described above
actually cause a quite distorted output signal as the actual values
probably alternates closely around zero.

[0213]FIG. 5C illustrates the output signal OS established by adding the
input signal IS and the outband signal OBS. As the input signal is zero,
the output signal is actually equal to the outband signal. The outband
signal in this example is, if in the continuous domain, a triangular
waveform with a frequency of half the frequency of the PWM periods
denoted by the divisions of the timeline. For a system sampling the
signal in the middle of each PWM period, the output signal will always be
outside the predetermined amplitude range between LT and HT. If the
sample time is shifted, e.g. by sampling at the beginning of each PWM
period, the outband signal may simply be shifted accordingly. It noted
that the outband signal may take any of several different forms and still
be sampled with the same result. Such forms comprise, e.g., saw-tooth
waveforms, sine waveforms, square waveforms, etc.

[0214]FIG. 5D illustrates the result of pulse width modulating the output
signal OS as shown in FIG. 5C. It is easily understood that this signal,
if the frequency is high enough, is rendered as silence by e.g. a
speaker.

[0215]In a typical application where the subsequent stages comprise power
switches for digital amplification and demodulation filtering only a
small fraction of the energy of the high frequency content is transformed
into heat. Thereby, the additional power consumption caused by adding the
outband signal is in practice insignificant.

[0216]FIG. 5E illustrates a second possible input signal IS. The further
features of FIG. 5E are equal to those of FIG. 5B and described with
reference to that figure. The input signal example of FIG. 5E lies within
the probably problematic range, i.e. between low threshold LT and high
threshold HT for approximately three successive PWM periods around the
centre of the time period shown.

[0217]FIG. 5F shows the resultant output signal OS when adding the input
signal of FIG. 5E with the outband signal OBS. The outband signal is in
this example equal to the outband signal used for rendering the output
signal of FIG. 5C. As seen, some of the problematic amplitudes of the
input signal of FIG. 5E have been pushed outside the problematic range
but in the same way some of the safe amplitudes have been pushed into the
problematic range between the LT and HT values.

[0218]Like FIG. 5D FIG. 5G illustrates the result of pulse width
modulating the output signal OS of FIG. 5F. The asterisks indicate pulses
that are too narrow, i.e. less than the minimum pulse width established
from the output signal amplitudes within the problematic range.

[0219]The embodiment of FIG. 5A may, thus, for some input signals IS cause
the level of the amplitude distribution function of the output signal OS
to be lower within the range between the low and high thresholds than the
level of the distribution function of the input signal IS, whereas it for
other input signals may worsen the problem. If the nature and
characteristics of the input signal IS are well defined or known, the
embodiment of FIG. 5A may be used to significantly reduce the number of
e.g. too narrow pulses in an PWM amplifier application.

[0220]Even if the established output signal causes a number of too narrow
pulses to be established, these will typically by means of the present
invention occur a little before or after the time where the signal should
be silent, i.e. a little before or after the zero-crossings of the input
signal. This effectively causes the distortion to be less significant as
the signal/error-ratio increases rapidly when moving away from the
zero-crossings. In other words, the unavoidable distortions are moved to
situations where the utility signal masks them.

[0221]FIG. 6A illustrates a further, more preferred embodiment of the
present invention.

[0222]Like the embodiment of FIG. 5A it comprises an input signal IS,
which is turned into an output signal OS by adding an outband signal OBS
to it by means of a summing point 62. The outband signal of the
embodiment of FIG. 6A is established by means of a level controlled
generator LCG. The level controlled generator is controlled at least
partly by an input connected to the input signal. An outband signal that
is amplitude controlled by the input signal may, thus, be established.
The particular characteristics of the outband signal, e.g. waveform,
frequency, etc., may be freely chosen or determined by other parameters
as with the embodiment of FIG. 5A, and need not to be controlled by the
input signal. The level controlled generator may, e.g., be implemented as
a signal generator with voltage controlled gain.

[0223]The embodiment of FIG. 6A further comprises a loop filter LF. The
loop filter is in the embodiment of FIG. 6A located in the feedback path
from the output signal OS in such a way that is processes the difference
between the output signal OS and the input signal IS and the output of
the loop filter LF is subtracted from the input signal IS by means of a
summing point 61. Assuming a linear loop filter LF and a silent outband
signal OBS, the filter and feedback loop causes the following
relationship between the output and the input signals:

OS=IS-LF(OS-IS),

which may be rearranged into:

O S I S = 1

[0224]Thus the loop filter and the feedback loop as arranged in the
embodiment of FIG. 6A does not have any effect on the input signal to
output signal relation, i.e. it has a gain of one.

[0225]However, for the relationship between the output signal and any
signal, distortion, error or noise injected to the forward path between
the summing point 61 and the feedback path starting point, in the
following referred to as e, the loop filter causes the following:

OS=IS+e-LF(OS-IS),

which, when ignoring the input signal IS, may be rearranged into:

O S e = 1 1 + L F

[0226]Thus, the loop filter LF applies to the error signal in a negated
way and may by means of a preferred filter characteristic serve as a
noise shaper to distortion and errors introduced to the signal forward
path. Such preferred loop filter implementation may comprise a low-pass
IIR filter, i.e. infinite impulse response filter, with a corner
frequency at the top of the desired utility band, e.g. 20 kHz, and a
large gain within the utility band. To the error e this implementation
causes any utility band content to be heavily attenuated.

[0227]Regarding the embodiment of FIG. 6A, addition of the level
controlled outband signal causes introduction of errors because the
outband signal depends on the input signal amplitude. The errors
introduced comprise, e.g., amplitude modulation side bands that also
affect the utility frequency band, e.g. the audio band. Due to the errors
being introduced within the noise-shaper loop, these errors are however
suppressed by that.

[0228]Regarding the level controlled generator LCG any rules or algorithms
for determining the outband signal amplitude on the basis of the input
signal may be implemented. Such a rule may, in a simple example, comprise
always letting the outband signal amplitude exceed the amplitude of the
input signal by an amount corresponding to just a little more than the
half of the range between the low threshold LT and the high threshold HT,
i.e. a safe offset SO. Thereby, for a zero input, the outband signal
amplitude stays just outside the range between the thresholds at the
sample times, provided that the frequency of the outband signal is half
the frequency of the sampling performed in subsequent stages. For a
non-zero input, the outband signal amplitude alternates between the safe
offset value SO and values considerably higher than the input signal
amplitude. Thus, this implementation requires far better dynamic
parameters than needed to provide for the input signal itself.

[0229]A preferred embodiment of the level controlled generator LCG
amplitude-determining algorithm is illustrated graphically in FIG. 6B.
This illustration comprises a horizontal axis corresponding to the
absolute of the input amplitude and a vertical axis corresponding to the
absolute of the outband signal amplitude, i.e. the output of the level
controlled generator. The half of the range between the low threshold and
the high threshold is shown as a horizontal dashed line HF. The amplitude
of the input signal IS is shown as a dashed line and the amplitude OSL of
the outband signal OBS is shown as a solid line. For a first input signal
amplitude range, i.e. input amplitudes below a low level LL, the outband
signal amplitude is controlled as described above simply by adding a safe
offset SO to the value of the input signal. Thereby, as described above
it is ensured that the output signal OS at the sample times is either the
safe offset SO or the twice of the input signal amplitude plus the safe
offset value. For a second input signal amplitude range, i.e. input
amplitudes between the low level LL and a middle level ML, the outband
signal amplitude is smoothly decreased by determining it as the
difference between a predetermined value, e.g. equal to the middle level
value, and the input signal amplitude. Within this second input amplitude
range the outband signal amplitude, thus, decreases in order to not get
clipped at the maximum amplitude level. For a third input signal
amplitude range, i.e. input amplitudes between the middle level ML and
maximum, the outband signal amplitude is steady at zero, i.e. the outband
signal is switched off. Within this third amplitude range the input
signal is, thus, not regulated in this specific embodiment of the present
invention.

[0230]The effect of the level controlled generator LCG of FIG. 6A for a
silent signal as illustrated in FIG. 5B is exactly like for the
embodiment of FIG. 5A. As the input signal is zero all the time, the
outband signal amplitude takes the value of the safe offset SO shown in
FIG. 6B. Hence, the output signal OS established by summing together the
silent input signal IS with the outband signal OBS, thus, becomes the
same as in FIG. 5C, where the signal is outside the problematic range at
every sample time. Obviously, the possible outcome illustrated in FIG. 5D
of pulse width modulating the output signal OS of FIG. 5C also applies to
this situation. None of the pulses are too narrow.

[0231]FIG. 6C illustrates a different input signal IS, equal to the signal
illustrated in FIG. 5E. It sweeps from a value above the high threshold
HT across zero to a value below the low threshold LT. FIG. 6D illustrates
the output signal OS resulting from summing together the input signal of
FIG. 6C with an outband signal established according to the algorithm
illustrated by FIG. 6B. The outband signal actually causes all of the
input signal amplitudes within the problematic range to be pushed outside
it, but nevertheless also causes a couple of good input signal amplitudes
to be moved into the problematic range. A result of pulse width
modulating the output signal is shown in FIG. 6E. As seen, two of the
pulses marked with asterisks are narrower than minimum pulse width.

[0232]FIGS. 7A to 7I illustrate the effect of the level controlled
generator LCG generated outband signal OBS of a preferred embodiment of
the present invention. In FIG. 7A an input signal IS is shown. The
horizontal axis corresponds to time or samples, whereas the vertical axis
corresponds to signal amplitude. FIG. 7A further comprises dashed lines
indicating the low level LL and middle level ML amplitudes corresponding
to FIG. 6B.

[0233]FIG. 7B comprises a possible outband signal OBS established from the
input signal of FIG. 7A on the basis of an algorithm in principle
corresponding to the one shown in FIG. 6B. From the beginning, i.e. the
left side of FIG. 7A, the input signal IS is above the middle level ML
and thus, according to the algorithm of FIG. 6B, the outband signal in
FIG. 7B is kept at zero. When the input signal decreases below the middle
level ML, the algorithm of FIG. 6B causes the outband signal to build up,
e.g. with an amplitude being the difference between the middle level ML
or another fixed level and the input signal IS. Furthermore, when the
input signal decreases below the low level LL, the algorithm causes the
amplitude of the outband signal OBS to track the input signal amplitude
at an offset, e.g. a safe offset SO.

[0234]FIG. 7C illustrates a possible output signal OS established by
summing together the input signal IS of FIG. 7A and the outband signal
OBS of FIG. 7B. Furthermore, the illustration comprises dashed lines
indicating the high threshold HT and the low threshold LT. As seen, the
output signal equals the input signal through the first stage, where the
input signal is above the middle level ML. In the next stage, between the
middle level and low level LL, the oscillation builds up until the lower
amplitude peaks are below the low threshold LT. From that time the lower
amplitude peaks stays just below the low threshold LT while the higher
amplitude peaks decrease with the signal until just above the high
threshold HT. From there, the opposite behaviour is seen for the rest of
the signal.

[0235]The outband signal OBS should, preferably, have a mean value of zero
in order not to disturb the utility signal. Thereby, it is achieved that
the output signal of FIG. 7C render as the input signal in FIG. 7A when
processed by a low pass filter. Furthermore, it may be seen that no
peaks, i.e. samples when the signal is used for e.g. a PWM amplifier, are
within the problematic range around the time where the input signal
crosses zero. The output signal, however, comprises altogether six
samples within the problematic area at the times where the outband signal
fades in or out. The distortion established by these samples is, however,
less significant as the higher input signal level at that time causes a
significantly higher signal/error-ratio.

[0236]FIGS. 7D to 7I correspond to FIGS. 4A to 4F but regarding the
embodiment of FIG. 6A instead of an empty amplitude distribution filter
ADF. Thus, the input signal IS used for establishing the FIGS. 7D to 7I
is a 1 kHz sine wave. FIG. 7D is a three-dimensional presentation of the
output level distribution, indicated by shades of grey, for different
input peak levels at the vertical axis and different output levels at the
horizontal axis. Darker shades indicate a higher concentration of a
specific output level within an output signal established from an input
signal with a specific input peak level.

[0238]It may be seen that for small input signals having a peak amplitude
of less than approximately 0.23, the output level distribution is zero
within the problematic range between the low threshold and high
thresholds, in this example approximately -0.05 and 0.05. This is because
the input signal at these levels never becomes above the low level LL
and, thus, never causes the outband signal OBS to decrease, cf FIG. 6B.

[0239]For higher input signal peak levels, i.e. above approximately 0.23,
some occurrences of output levels within the problematic range appear.
This is due to the transition of the outband signal OBS from the low
level to the middle level.

[0240]FIG. 8A illustrates an alternative embodiment of the amplitude
distribution filter ADF of the present invention. It comprises an input
signal IS and an output signal OS. The input signal is processed by a
mapping means MM in order to establish the output signal OS. The
embodiment, furthermore, comprises a loop filter LF as in the embodiment
of FIG. 6A. This loop filter is coupled in the same way as with the
former embodiment and serves the same purpose, namely noise shaping of
errors or noise injected to the forward path subsequent to the summing
point 81. The present embodiment also in a preferred embodiment comprises
a quantizing means QZ but also embodiments without a separate quantizing
means are within the scope of the present invention, hence the dashed
outline.

[0241]The mapping means MM maps any input level within the one or more
problematic ranges to levels outside the problematic ranges. To levels
already outside the problematic ranges nothing is done. The mapping means
may be kinds of hard limiters or any other means more or less intelligent
that facilitate clearance of a specific amplitude range.

[0242]A possible effect of such a mapping means MM is shown by FIGS. 8B
and 8C. FIG. 8B comprises an example input signal IS sweeping with time
from a high amplitude across zero and a problematic range between a high
threshold HT and a low threshold LT to a low amplitude. FIG. 8C
illustrates a possible output 83 of a mapping means MM when fed with the
input signal of FIG. 8B. It is noted that the effect of the mapping means
shown in FIG. 8C is for a stand-alone mapping means, i.e. without any
feedback compensation or noise-shaping. When the input signal decreases
below a value a little higher than the high threshold HT, the mapping
means keeps the output signal 83 at that value, until the input signal
decreases below zero. Then the mapping means maintains an output level of
a little below the low threshold LT until the input signal also decreases
below that value and the mapping means has its output 83 follow the input
signal.

[0243]As the mapping function 83 as illustrated in FIG. 8C, however,
introduces significant amounts of errors in the utility band, the noise
shaping feedback is applied by means of the loop filter LF. This filter
may comprise the same characteristics as the loop filter in FIG. 6A, i.e.
a low-pass IIR filter with a corner frequency at the top of the desired
utility band, e.g. 20 kHz, and a large gain within the utility band.

[0244]The quantizing means QZ may be added when any subsequent stages are
not able to exploit the amplitude resolution of the input signal.
Quantizing the signal before mapping it may ease the implementation of
the mapping means considerably. Furthermore, by placing the quantizing
means within the forward path being noise-shaped quantizing errors are
rejected. By placing the quantizing means QZ within the noise shaping
loop instead of subsequently to the amplitude distribution filter ADF, as
it is also possible, the errors established by the quantizing means QZ
are also noise shaped.

[0245]FIGS. 9A to 9H illustrate possible effects of the embodiment of FIG.
8A. FIG. 9A comprises a possible input signal sweeping with time from its
maximum level 1 to its minimum level -1. FIG. 9B illustrates a possible
output signal OS established by the embodiment illustrated in FIG. 8A,
i.e. the combined mapping means MM and loop filter LF. The mapping means
MM has caused the output signal to comprise no peak amplitudes within the
range between the low threshold LT and high threshold HT. The oscillating
nature of the output signal around the time where the input signal
crosses zero, as opposed to the straightforward mapping shown in FIG. 8C,
is caused by the noise-shaping loop.

[0246]FIGS. 9C to 9H correspond in type to the FIGS. 4A to 4F and 7D to 7I
but are established by inputting a 1 kHz sine wave to the embodiment of
FIG. 8A. As seen, the output level distribution is zero within the
problematic range for any input peak level, i.e. no pulses of less than
the minimum pulse width ever occur when applying the present embodiment
to a PWM amplifier. Regarding FIG. 9D illustrating the output level
distribution for a zero input signal, i.e. when idling, a certain
concentration of output levels below and above the low and high
thresholds exists. This is due to the noise shaping mechanism causing
several different amplitude levels to be mapped to instead of just the
threshold levels.

[0247]FIG. 10 illustrates a preferred embodiment of the amplitude
distribution filter ADF of the present invention. It comprises an input
signal IS and an output signal OS. Furthermore, it comprises the elements
of both the embodiment of FIG. 6A and the embodiment of FIG. 8A, i.e. a
level controlled generator LCG adding an outband signal OBS to the input
signal, a mapping means MM mapping signal level within certain ranges to
certain different levels, and a loop filter LF, which in virtue of its
location performs noise shaping.

[0248]The addition of the outband signal to the input signal causes the
input signal, at times with an amplitude close to zero, to be oscillating
around the problematic range but, nevertheless, sometimes causes
amplitudes within the problematic range to occur at other times. By
letting this composite signal comprising the outband signal and the input
signal to be processed by the mapping means MM, it is ensured that no
output signal levels within the problematic range occur, i.e. the output
level distribution is zero between the low and high thresholds. The
distortion injected by the mapping means is less than when the signal is
not pre-processed by adding the outband signal as the outband signal
causes far less samples to be mapped and also causes the times where
mappings are required to be moved farther away from the input signal zero
crossings.

[0249]FIGS. 11A to 11C illustrate how the embodiment of FIG. 10 may affect
a possible input signal. A possible input signal IS is shown in FIG. 11A
together with dashed lines indicating the low level LL and middle level
ML used for establishing the outband signal according to the algorithm of
FIG. 6B. A corresponding outband signal OBS is shown in FIG. 11B. FIG.
11C illustrates a possible output signal OS established by the embodiment
illustrated in FIG. 10 and on the basis of the input signal of FIG. 11A
and the outband signal of FIG. 11B. FIG. 11C, furthermore, comprises
dashed lines indicating a low threshold LT and a high threshold HT. As it
can be seen, it is possible by means of the embodiment of FIG. 10 to
establish an output signal comprising no sample values within the
problematic range between the low and high thresholds. As described above
this results from the combined work of the level controlled generator and
summing point 102, the mapping means MM, and the noise shaping loop
filter LF.

[0250]FIGS. 11D to 11I illustrate examples of the output signal level
distribution that may result from the embodiment of FIG. 10. FIG. 11D to
11I correspond in type to e.g. the FIGS. 9C to 9H but are established by
inputting a 1 kHz sine wave to the embodiment of FIG. 10. As seen, the
output level distribution is zero within the problematic range for any
input peak level, i.e. no pulses of less than the minimum pulse width
ever occur when applying the present embodiment to a PWM amplifier.

[0251]FIGS. 12 and 13 illustrate the relative noise floor, i.e. distortion
and errors, which is comprised by the output signal by using the
amplitude distribution filter embodiments of FIGS. 8A and 10,
respectively. The horizontal axes correspond to the absolute input level,
i.e. 0 to 1 for the examples given above. The vertical axes correspond to
the relative noise floor of the output signal. By comparing the two
illustrations it is easily seen that the embodiment of FIG. 10 that
combines a level controlled generator with mapping means adds far less
noise to small signals, i.e. input levels less than approximately 0.2,
than the embodiment of FIG. 8A comprising only mapping means. As noise in
small signals is naturally more disturbing than in large signals, the
embodiment of FIG. 10 is preferred.

[0252]FIGS. 14A to 14D illustrate an alternative embodiment of the present
invention. FIG. 14A is identical to FIG. 6A and comprises a level
controlled generator establishing an outband signal OBS on the basis of
an input signal IS and a loop filter LF acting as a noise shaper. FIG.
14B is identical to FIG. 5B and shows an empty input signal IS and low
threshold LT and high threshold HT. FIG. 14C illustrates a possible
output signal OS established by the embodiment of FIG. 14A. As the input
signal IS is empty, the output signal OS of FIG. 14C also corresponds to
the outband signal OBS added by the summing point 142. Any preferences
and characteristics of the outband signal mentioned above, e.g. regarding
FIG. 5A to 6E, also apply to the outband signal of the present
embodiment, however, the outband signal of the present embodiment is
preferably of a frequency rate a quarter of the rate at which the output
signal is sampled in subsequent stages, e.g. in a subsequent pulse width
modulator or digital amplifier. If, for example, the switching rate in a
subsequent PWM amplifier is 384 kHz, the rate of a preferred outband
signal would be 96 kHz. Thereby, it is possible as shown in FIG. 14C to
have every second sample value to be zero for an empty input signal.

[0253]As described above, the present example assumes a problematic range,
e.g. due to non-linear processing in subsequent stages between the low
threshold LT and high threshold HT. However, as also described above, the
level of exact zero is in fact not typically problematic. In above
described embodiments, it is just more uncomplicated to treat the special
value of zero as belonging to the problematic range. For applications
where zero is in fact not a problem to process correctly, e.g. in a PWM
system where zero is represented by no pulse at all, i.e. a duty cycle of
0%, the problematic range may as well be divided into a range from the
low threshold LT to, but not including, zero and a range from, but not
including, zero to the high threshold HT. In such a system the embodiment
of FIG. 14C may cause significant power savings.

[0254]As for an empty input signal every second output sample is zero, the
power switches in a possible subsequent PWM amplifier should only produce
the half of the pulses of a typical amplifier as also illustrated in FIG.
14D. Thereby, also electromagnetic compatibility and interference
(EMC/EMI) issues may be improved.

[0255]FIGS. 15A to 15C illustrate how the embodiment of FIG. 14A may
process an example input signal not being zero. FIG. 15A illustrates a
possible input signal IS, sweeping from 1 to -1. It also illustrates
possible low level LL and middle level ML of a possible level controlled
outband signal algorithm as, e.g., the one illustrated in FIG. 6B. FIG.
15B illustrates a possible outband signal OBS established by the
embodiment of FIG. 14A. The preferred sample values are shown by dots.
FIG. 14C illustrates a possible output signal OS established by summing
together the signals of FIGS. 14A and 14B and applying noise shaping by
means of the loop filter LF. The preferred sample values are shown by
dots. Also the low threshold LT and high threshold HT are shown. It can
be seen that several sample values are within the problematic range, but
not zero, i.e. requires impossible narrow pulses to be established if
subsequently processed by a typical PWM amplifier. And, unfortunately,
several of these values occur at times where the input signal, i.e.
utility content of the output signal, is close to zero.

[0256]FIGS. 15D to 15I illustrate examples of the output signal level
distribution that may result from the embodiment of FIG. 14A. FIG. 15D to
15I correspond in type to, e.g., the FIGS. 9C to 9H but are established
by inputting a 1 kHz sine wave to the embodiment of FIG. 14A. As seen
from FIGS. 15D and 15E only output levels of zero and safe offsets to
either side of zero occur. But as seen from FIGS. 15F to 15I and, e.g.,
by comparison with FIGS. 7F to 7I, the concentration of occurrences of
levels within the problematic range and, thus, pulses of less than the
minimum pulse width if used for a PWM amplifier is greater than with the
embodiment of FIG. 6A.

[0257]FIGS. 16A to 16C illustrate yet an alternative embodiment of the
present invention. FIG. 16A is identical to FIG. 8A and comprises an
input signal being processed by an optional quantizing means QZ, a
mapping means MM and a loop filter LF. Contrary to the embodiment of FIG.
8A, the mapping means MM of the present embodiment maps values within the
problematic range to zero instead of outside the problematic range. This
is illustrated in FIGS. 16B and 16C. FIG. 16B illustrates a possible
input signal IS and the problematic range is indicated by low threshold
LT and high threshold HT. FIG. 16C illustrates the working of the mapping
means MM by showing what would be the output 163 of the mapping means MM
if the loop filter LF and feedback path were omitted. As seen from FIG.
16C the output 163 is mapped to the zero level when the input decreases
below a value just above the high threshold HT and stays at zero until
the input decreases below a value just below a low threshold LT.

[0258]The quantizing means QZ may be added when any subsequent stages are
not able to exploit the amplitude resolution of the input signal.
Quantizing the signal before mapping it may ease the implementation of
the mapping means considerably. Furthermore, by placing the quantizing
means within the forward path being noise-shaped results in the rejection
of quantizing errors.

[0259]FIGS. 17A to 17H illustrate possible effects of the embodiment of
FIG. 16A. FIG. 17A comprises a possible input signal sweeping with time
from its maximum level 1 to its minimum level -1. FIG. 17B illustrates a
possible output signal OS established by the embodiment illustrated in
FIG. 16A, i.e. the combined mapping means MM and loop filter LF. The
mapping means MM has caused the output signal to comprise several
preferred sample points, indicated by dots, at the zero level but no
points at other levels within the problematic range, i.e. the range
between the low threshold LT and high threshold HT. The noise-shaping
loop causes a few points to be pushed outside the problematic range
instead of being mapped to zero.

[0260]FIGS. 17C to 17H correspond in type to, e.g., the FIGS. 9C to 9H but
are established by inputting a 1 kHz sine wave to the embodiment of FIG.
16A. As seen, the output level distribution has a high concentration at
the output level of zero all input peak levels. Moreover, the output
level distribution is zero for all other values within the range between
the low threshold and high threshold, i.e. no pulses of less than the
minimum pulse width, except for zero, ever occur when applying the
present embodiment to a PWM amplifier. Regarding FIG. 17D illustrating
the output level distribution for a zero input signal, i.e. when idling,
a certain concentration of output levels below and above the low and high
thresholds exists.

[0261]This is due to the noise shaping mechanism, sometimes causing
different amplitude levels outside the problematic range to be mapped to
instead of just zero.

[0262]FIG. 18 illustrates a preferred embodiment of the amplitude
distribution filter ADF of the present invention. It comprises an input
signal IS and an output signal OS. Furthermore, it comprises the elements
of both the embodiment of FIG. 14A and the embodiment of FIG. 16A, i.e. a
level controlled generator LCG adding an outband signal OBS to the input
signal, a mapping means MM mapping signal level within certain ranges to
zero and a loop filter LF, which in virtue of its location performs noise
shaping.

[0263]The addition of the outband signal to the input signal causes the
input signal, at times with an amplitude close to zero, to be oscillating
around the problematic range but, nevertheless, sometimes causes
amplitudes within the problematic range to occur at other times.
Furthermore, for empty input signals it causes every second preferred
sample point to be at a level of zero. By letting this composite signal
comprising the outband signal and the input signal to be processed by the
mapping means MM it is ensured that no output signal levels within the
problematic range, except for zero, occur, i.e. the output level
distribution is zero between the low and high thresholds except for the
output level zero, which has a high concentration. The distortion
injected by the mapping means is reduced compared to an embodiment not
proving an additive outband signal, as the outband signal causes less
samples to require mapping and also causes the situations where mapping
are required to be moved farther away from the input signal zero level
crossings. The signal/error-ratio is thus improved for small signals
compared to the signal/error ratio of, e.g., the embodiment of FIG. 16A.

[0264]FIGS. 19A to 19C illustrate how the embodiment of FIG. 18 may affect
a possible input signal. A possible input signal IS is shown in FIG. 19A
together with dashed lines indicating the low level LL and middle level
ML used for establishing the outband signal according to the algorithm of
FIG. 6B. A corresponding outband signal OBS is shown in FIG. 19B with
preferred sample points indicated by dots.

[0265]FIG. 19C illustrates a possible output signal OS established by the
embodiment illustrated in FIG. 18 and on the basis of the input signal of
FIG. 19A and the outband signal of FIG. 19B. Furthermore, FIG. 19C
comprises dashed lines indicating a low threshold LT and a high threshold
HT and dots indicating preferred sample points. As it can be seen, it is
possible by means of the embodiment of FIG. 18 to establish an output
signal comprising only sample values outside the problematic range
between the low and high thresholds and sample values with a level of
zero. Thereby, it is ensured that a possible subsequent PWM amplifier is
not required to establish pulses with less than a minimum pulse width.

[0266]FIGS. 19D to 19I illustrate examples of the output signal level
distribution that may result from the embodiment of FIG. 18. FIGS. 19D to
19I correspond in type to, e.g., the FIGS. 9C to 9H, but are established
by inputting a 1 kHz sine wave to the embodiment of FIG. 18. As seen, the
output level distribution is zero within the problematic range for any
input peak level except for the output level of zero, which represents a
high concentration. Thus, it can be seen that the embodiment of FIG. 18
combining the embodiments of FIGS. 14A and 16A improves the performance
of each of these.

[0267]FIGS. 20 and 21 illustrate the relative noise floor, i.e. distortion
and errors that may be comprised by an output signal when using the
amplitude distribution filter ADF embodiments of FIGS. 16A and 18,
respectively. The horizontal axes correspond to the absolute input level,
i.e. 0 to 1 for the examples given above. The vertical axes correspond to
the relative noise floor of the output signal. By comparing the two
illustrations it is easily seen that the embodiment of FIG. 18 that
combines a level controlled generator with mapping means adds far less
noise to small signals, i.e. input levels less than approximately 0.02,
than the embodiment of FIG. 8A comprising only mapping means. As noise in
small signals is naturally more disturbing than in large signals the
embodiment of FIG. 18 is preferred.

[0268]It is noted that the outband signal of the embodiments of FIGS. 14A
and 18 should preferably cause a significant amount of zeroes to be
sampled by subsequent stages. When the outband signal is a periodic
signal, the amount of zeroes to be sampled should, preferably, be less
than the half of the samples and, most preferably, the half of the
samples, e.g. every second sample. Also non-periodic signals, e.g. white
noise, may be used for outband signals but should also be established in
a way ensuring a considerable amount of zeroes to be sampled.

[0269]FIG. 22A illustrates a further, preferred embodiment of the present
invention. Like the embodiment of FIG. 6A it comprises an input signal
IS, which is turned into an output signal OS by adding an outband signal
OBS to it by means of a summing point 222. The outband signal of the
embodiment of FIG. 22A is established by means of a level controlled
generator LCG. The level controlled generator is controlled at least
partly by an input connected to the input signal. An outband signal that
is amplitude controlled by the input signal may, thus, be established.
The particular characteristics of the outband signal, e.g. waveform,
frequency, etc., may be freely chosen or determined by other parameters
as with the embodiment of FIG. 5A, and need not to be controlled by the
input signal. The level controlled generator may, e.g., be implemented as
a signal generator with voltage controlled gain.

[0270]The embodiment of FIG. 22A further comprises a loop filter LF. The
loop filter is in the embodiment of FIG. 6A located in the feedback path
from the output signal OS in such a way that is processes the difference
between the output signal OS and the input signal IS and the output of
the loop filter LF is subtracted from the input signal IS by means of a
summing point 221.

[0271]Thus, the loop filter LF as described regarding the embodiment of
FIG. 6A serves as a noise shaper to distortion and errors introduced to
the signal forward path. Such preferred loop filter implementation may
comprise a low-pass IIR filter, i.e. infinite impulse response filter,
with a corner frequency at the top of the desired utility band, e.g. 20
kHz, and a large gain within the utility band.

[0272]Regarding the level controlled generator LCG any rules or algorithms
for determining the outband signal amplitude on the basis of the input
signal may be implemented. As described above with reference to FIG. 6A
the choice of algorithm or rule to implement may be restricted by the
system dynamics, in particular typically requiring the output signal OS
to not exceed the maximum level of the input signal, e.g. 1, and thus
requiring the sum of the input signal IS and the outband signal OBS to
not exceed e.g. 1.

[0273]The typical restrictions described above may however not always
apply, as e.g. when the subsequent processing means comprises certain
kinds of three-level PWM amplifiers. An example of such an amplifier
typically comprises an H-bridge system or a bridge-tied load system,
where a single power supply with +VCC and GND by means of typically 4
switches are able to establish three levels, +VCC, GND and -VCC over the
load. Typically positive input signal values are represented by means of
pulses of level +VCC established by a first set of 2 switches, whereas
negative input signal values are represented by means of pulses of level
-VCC established by a second set of 2 switches. Between the pulses the
level is GND. The load should be coupled between the outputs of the two
sets of switches, instead of as typically between one set of switches and
the ground-plane. Hence, if the input signal is, e.g., a positive
DC-signal only the first set of switches operates, and vice versa.

[0274]In such a system, if it can be ensured that the signal to amplify
always alternates between positive and negative levels in synchronism
with the switch frequency, there will always for each set of switches
between the active switch periods be an empty period where only the other
set of switches is working. In this situation it may be possible to let
the pulse widths increase beyond the dedicated periods into the preceding
and succeeding empty periods. Thereby it is possible to establish pulses
of twice the width of the otherwise maximum pulse width, and thereby
significantly improve the dynamics of the system. When it is furthermore
ensured that the two sets of switches do not operate concurrently, e.g.
by ensuring that the sum of the absolute of two successive signal levels
does not exceed, e.g., 2, the system is further improved, e.g. in that
the two sets of switches do not distort the operation of each other. It
is noted that the use of this concept in alternative ways or systems for,
e.g., further improving the dynamics is within the scope of the present
invention.

[0275]In a system as described above providing improved dynamics, in
particular doubling of the maximum possible amplitude, the level
controlled generator LCG of FIG. 22A may comprises level control
algorithms that is not possible to use with the embodiment of FIG. 6A. An
example of such a preferred algorithm is illustrated graphically in FIG.
22B. This illustration comprises a horizontal axis corresponding to the
absolute of the input amplitude and a vertical axis corresponding to the
absolute of the outband signal amplitude, i.e. the output of the level
controlled generator. The half of the range between the low threshold and
the high threshold, i.e. typically the range from zero level to the high
threshold, is shown as a horizontal dashed line HT. The amplitude of the
input signal IS is shown as a dashed line and the amplitude OSL of the
outband signal OBS is shown as a solid line. The outband signal amplitude
is for all input levels determined by adding a safe offset SO to the
value of the input signal. Thereby, for a zero input, the outband signal
amplitude stays just outside the range between the thresholds at the
sample times, provided that the frequency of the outband signal is half
the frequency of the sampling performed in subsequent stages. For a
non-zero input, the outband signal amplitude alternates between the safe
offset value SO and values considerably higher than the input signal
amplitude. Thus, this implementation requires far better dynamic
parameters than needed to provide for the input signal itself, and such
dynamics are just what is provided by the above described PWM amplifier.

[0276]The effect of the level controlled generator LCG of FIG. 22A for a
silent signal as illustrated in FIG. 5B is exactly like for the
embodiment of FIG. 5A. As the input signal is zero all the time, the
outband signal amplitude takes the value of the safe offset SO shown in
FIG. 22B. Hence, the output signal OS established by summing together the
silent input signal IS with the outband signal OBS, thus, becomes the
same as in FIG. 5C, where the signal is outside the problematic range at
every sample time. Obviously, the possible outcome illustrated in FIG. 5D
of pulse width modulating the output signal OS of FIG. 5C also applies to
this situation. None of the pulses are too narrow.

[0277]FIG. 22C illustrates a different input signal IS, first constant at
approximately -0.8, then ramping to approximately 0.8 where it stays. The
horizontal axis corresponds to time, and vertical strokes indicate the
switch periods. The vertical axis corresponds to the signal amplitude.
Horizontal lines are provided corresponding to the low threshold LT and
high threshold HT. The diagram further comprises a possible output signal
OS established by means of the embodiment of FIG. 22A on the basis of the
input signal IS and the level control algorithm of FIG. 22B. When the
input signal level is negative, the output signal alternates between a
positive value corresponding to the safe offset SO, just above the high
threshold HT, and a negative value corresponding to twice the input
signal level minus the safe offset SO. When the input signal level is
positive, the output signal alternates between a negative value
corresponding to the negative of the safe offset SO, just below the low
threshold LT, and a positive value corresponding to twice the input
signal level plus the safe offset SO. Hence, when the input signal
approaches the maximum amplitude of 1, the output signal approaches twice
that amplitude, i.e. 2 in the present example. By using the algorithm of
FIG. 22B is obtained that no output signal samples is between the low
threshold and high threshold.

[0278]FIG. 22C furthermore comprises examples of a PWM signal established
from the output signal OS of FIG. 22C. Strokes on the horizontal axes
indicate the PWM periods. The PWM signal in this example is a three level
signal comprising a first signal PWMA and a second signal PWMB. The
signals are established in such a way that positive output signal levels
are represented by pulses in the first signal PWMA, and negative output
signal levels are represented by pulses in the second signal PWMB. When
signal represented by the PWM signal alternates between negative and
positive values, as the output signal in FIG. 22C, there will always be
an empty PWM period between each pulse in each signal. As described above
these empty periods may be used for increasing the pulse widths of the
active periods. Thereby it is possible to establish a three level PWM
signal representing the output signal of FIG. 22C, i.e. an output signal
normally exceeding the system dynamics. The present embodiment of the
invention furthermore ensures that the two PWM signal parts PWMA and PWMB
preferably never, and at least less often, comprise pulses at the same
times, by ensuring narrow, yet not too narrow, pulses in one signal part,
e.g. PWMA, while establishing broad pulses in the other signal part, e.g.
PWMB.

[0279]Regarding the embodiment of FIG. 22A and the above description
should be noted that also three-level PWM techniques and H-bridge
implementations that do not behave as the examples given above are known.
A conventional three-level differential PWM technique where two sets of
switches operates simultaneously thus establishing overlapping pulses is
an example of a such, which however typically provides poor
electromagnetic compatibility and interference (EMC/EMI) properties as
well as higher power dissipation.

[0280]FIGS. 23A and 23B illustrate relationships between the output and
the input of a process being non-linear for inputs within certain ranges.
They relate to FIGS. 3A and 3B illustrating the ideal relationship and a
relationship comprising a non-linear processing range in the middle of
the full level range. FIG. 23A comprises an example of a relationship 231
comprising several amplitude ranges in which the output is distorted,
whereof also some of these obviously does not comprise the zero level.
The ranges comprise values between a first low threshold LT1 and a first
high threshold HT1, between second low and high threshold LT2, HT2, and
between third low and high thresholds LT3, HT3.

[0281]FIG. 23B illustrates an output/input relationship 232 where the
amplitude range with non-linear relationship is dynamically positioned
within the input level range. As indicated by arrows the range extends
from a dynamically positioned low threshold DLT to a dynamically
positioned high threshold DHT. It is noted that both the extent of the
range as well as the position may in some embodiments be dynamically
adapted. It is furthermore noted that a possible output/input
relationship may simultaneously comprise several dynamically positioned
ranges or a combination of fixed and dynamically positioned ranges, and
that also the number of ranges may be dynamically changed, e.g. when two
or more ranges partly or fully coincide, or due to any other reason.

[0282]An example of an application where dynamically positioned
problematic amplitude ranges as described above regarding FIG. 23B may be
advantageously utilized is given in FIG. 24. It comprises an embodiment
of a multi-channel PWM modulator system MCS. Such a system may, e.g., be
used for pulse width modulating several audio channels, e.g. 6 channels,
and may advantageously be implemented in a single integrated circuit. One
of several issues to consider when implementing a system in an integrated
circuit is the use of external chip connectors, as the number of these
significantly impacts the cost of the integrated circuit, i.e. production
and materials. A possible solution to this problem is to combine the
multiple audio channels into a fewer number of physical conductors. When,
e.g., the system comprises 6 audio channels it may be possible by means
of a proper multiplexing algorithm, compression algorithm, etc., to
combine the information of these into, e.g., 2 or 4 physical wires. In
FIG. 24 is shown two signals entering the multi-channel PWM modulator
system MCS. These signals may each require more than one physical
connector, but use together preferably less than 6 connectors. Within the
multi-channel system MCS the combined channels signal is split into a
signal for each of the 6 individual channels by a signal splitter 121.
Alternatively each of the 6 channels may enter the multi-channel system
MCS by its own physical connector. Each of the 6 channels are provided to
a pulse width modulating system PWMS1, PWMS2, . . . , PWMS6 according to
the present invention as input signals, whereof due to clarity in the
drawing only a reference IS1 is given for the first channel. The
modulator output signals MOS1, etc., which are pulse width modulated
representations for the input signal IS1, etc., are again combined into
preferably less than 6 physically wired signals by means of a signal
combiner 123.

[0283]FIG. 24 further comprises a signal splitter 124 for dividing the
combined modulator output signal into a signal for each channel outside
the integrated circuit comprising the multi-channel pulse width modulator
system MCS. Each of these pulse width modulated output channels may then
be fed to, e.g., separate amplifiers AMP1, . . . , AMP6, preferably
switch-mode amplifiers. Alternatively the combined modulator output
signal may be fed directly to each of the subsequent, e.g., amplifiers by
bypassing the signal splitter 124. The subsequent stage, e.g. amplifier,
should then be adapted to retrieve from the combined signal only the
relevant channel.

[0284]In order to most optimally combine multiple pulse width modulated
signals MOS1, etc., into a fewer physical signals by means of signal
combiner 123 it may be beneficial to assume that none of the PWM signals
comprise concurrent pulse flanks. As the input signal amplitudes
determine the pulse widths, and thus the flanks of the pulses,
non-concurrent pulse flanks may be ensured by ensuring that a pulse width
modulator PMOD of one pulse width modulator system PWMS1, . . . , PWM6
never receive the same intermediate output signal OS amplitude at the
same time as the modulator of another system PWMS1, . . . , PWMS6, as
this would probably cause concurrent pulse flanks to be established.

[0285]A further reason for desiring non-concurrent pulse flanks is the
probability of establishing cross-talk when, e.g., the amplifiers AMP1, .
. . , AMP6 are operated from the same power supply. By ensuring that the
switches in different amplifiers are never required to switch
simultaneously, the problem of cross-talk may be reduced.

[0286]Guaranteeing or at least increasing the probability of
non-concurrent flanks in a multi-channel system, e.g. a stereo system or
a 5.1 system, is thus desired, and one way in which this may be ensured
is, within the example embodiment of FIG. 24, by dynamically adapting the
problematic amplitude ranges of some of the pulse width modulator systems
PWMS1, . . . , PWMS6 according to the intermediate output signal
amplitudes of other pulse width modulator systems PWMS1, . . . , PWMS6.

[0287]Such dynamically adapting the problematic amplitude ranges may e.g.
be performed be a control unit 122 connected with each pulse width
modulator system PWMS1, . . . , PWMS6 by means of two-way external
control signals ECS1, etc. Thereby the control unit 122 may continuously
obtain information of the currently processed input values or
intermediate output values, and adaptively establish control information
accordingly.